Laminate of metal sheets and polymer

ABSTRACT

The invention relates to a laminate of metal sheets and fiber-reinforced polymer layers connected thereto. The laminate comprises at least one thick metal sheet with a thickness of at least 1 mm that is connected to the rest of the laminate by means of at least one fiber-reinforced polymer layer, the fiber volume content of which is at most 45 volume-%. The invention also relates to a method for producing the laminate as well as skin sheets reinforced with the laminate for an aircraft or spacecraft.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/303,790, filed on Dec. 8, 2008, which is a national phase filingunder 35 U.S.C. 371 of International Application No. PCT/NL2007/050265,filed on Jun. 5, 2007, which claims the benefit of Netherlands PatentApplication 2000100, filed Jun. 13, 2006, the entireties of theseapplications are hereby incorporated herein by reference for theteachings therein.

FIELD

The invention relates to a laminate of metal sheets and fiber-reinforcedpolymer layers connected thereto. The invention also comprises theapplication of such a laminate as a reinforcing sheet for components ofan aircraft or spacecraft.

BACKGROUND

Moldings made of a laminate comprising at least one metal sheet and onefiber-reinforced polymer layer connected thereto (hereinafter referredto as a metal laminate, fiber metal laminate or laminate for short) areincreasingly used in industries such as the transportation industry, forexample in cars, trains, aircraft and spacecraft. Such laminates can forexample be used in the wings, fuselage and tail panels and/or other skinpanels for aircraft, and generally ensure an improved fatigue resistanceof the aircraft component.

The known fiber metal laminates were developed in the period between1978 and 1990, and were sold on the market under brand names Arall™ andGlare™. The known fiber metal laminate is constructed of a large numberof relatively thin (typically 0.2 mm to 0.4 mm thick) aluminum sheetswith polymer adhesive layers reinforced with aramid fibers (Arall™) orhigh strength glass fibers (Glare™) in-between. This means that thefiber volume content in the adhesive layers is relatively high withtypical values of approximately 50 volume-% for Arall™ and 60 volume-%for Glare™. Fiber metal laminates generally demonstrate good resistanceto crack growth. A fatigue crack that has appeared in a metal sheet whensubjected to alternating load will therefore not continue to growquickly but will decelerate in crack growth. According to currentknowledge, this is caused by the fiber-reinforced polymer layers—inparticular the fibers thereof—spanning the crack and at least partiallyabsorbing the forces responsible for crack growth.

Although the known fiber laminate demonstrates good fatigue propertiesfor fuselage and wing skins, for example in an aircraft, it turns outthat the production costs are relatively high. This is due to a numberof factors. Typical moldings, in particular aircraft wing skins, canhave a total thickness amounting to several centimeters. This means thattens of—or even a hundred—different layers are generally needed toconstruct a molding out of a fiber laminate. Furthermore, the thin metalsheets applied in the fiber laminate have to be rolled out to an extremeextent, to achieve the low thickness required for good fatigueproperties. The metal sheets and fiber-reinforced polymer layers alsohave to meet close tolerances with respect to their composition andthickness. In addition, in the known fiber laminate, all metal sheetshave to be treated so that they bond effectively to the fiber-reinforcedpolymer layers. Each metal sheet therefore has to be anodized and primedfor example. Finally, to produce a molding out of the known fiberlaminate, all layers are positioned on a mold. The more layers thestructure requires, the more time-consuming and therefore costly it isto produce the molding.

A large number of the above problems could be resolved by applyingthicker metal sheets in the known fiber metal laminate. However, ifcracks appear in these metal sheets, a greater load will inevitably betransferred to the fiber-reinforced polymer layers spanning the cracks.Although the known fiber-reinforced polymer layers perform theircrack-spanning function effectively in the known fiber laminate, theycause areas of delamination that are too considerable between thecracked metal sheet and adjacent fiber-reinforced polymer layer if themetal sheets are significantly thicker.

SUMMARY

The object of the invention is to provide a laminate of the typereferred to in the preamble, that can be used to meet the highrequirements set by the aviation and space industry more effectively,and that can also be produced cost-effectively. The laminate accordingto the invention is thereto characterized as referred to in claim 1. Ithas surprisingly been found that resistance to delaminationsignificantly increases if the laminate comprises at least one thickmetal sheet with a thickness of at least 1 mm, and if this thick metalsheet connects to the rest of the laminate by means of at least onefiber-reinforced polymer layer, the fiber volume content of which is atmost 45 volume-%. The laminate is thereby rendered more thansufficiently resistant to fatigue and can moreover be produced moresimply and cheaply than the known laminate. By means of the measuresdescribed in the main claim, it is possible to use thicker metal sheetsin the fiber metal laminates than has been the case to date. By applyingthicker metal sheets in the laminate according to the invention, thereis a greater chance of imperfections in the fiber impregnation. This isdue to the high bending stiffness of such sheets, causing these sheetsto create a pressure drop in the fiber-reinforced polymer layers of thelaminate when the laminate is cured in an autoclave for example, whichin turn impedes impregnation of the fibers contained therein. A furtheradvantage of applying at least one fiber-reinforced polymer layer havinga reduced fiber volume content is that there is less risk ofinsufficiently impregnated fibers in the fiber-reinforced polymerlayers. Despite the fact that the laminate according to the inventioncomprises one or more fiber-reinforced polymer layers having a reducedfiber volume content, the laminate possesses good mechanical properties.

In an embodiment, a laminate according to the invention is characterizedin that the fiber volume content of the specified fiber-reinforcedpolymer layer is at most 39 volume-%, at most 34 volume-%, at most 30volume-%. Such fiber volume contents are lower than the contents usuallyapplied in fiber-reinforced polymers. When reference is made in thisapplication to a fiber-reinforced polymer layer having a reduced fibervolume content, it is understood to be a layer having a fiber volumecontent of at most 45 volume-%, at most 39 volume-%, at most 34volume-%, at most 30 volume-%. The fiber-reinforced polymer layer havinga reduced fiber volume content can for example be achieved by using asemi-finished product in which the fibers in the specified volumecontent are impregnated with a suitable polymer in a partially curedstate (referred to as prepregs). It is also possible to combine aprepreg having a usual fiber volume content of 60 volume-% for example,with one or more polymer adhesive layers, in order to achieve an averagereduced fiber volume content. In such a case, an adhesive layer isapplied that is provided with a carrier, for example in the form of anetwork of polymer fibers, for example polyamid fibers. The carrierensures that the adhesive layer retains a specific, pre-set thicknesseven after adhesion and curing. This is also advantageous for resistanceto delamination. It is also possible according to the invention tocombine dry—i.e. non-impregnated—fibers with a polymer adhesive layer inthe appropriate volume ratios.

In a further embodiment, a laminate according to the invention includesat least one thick metal sheet having a thickness of at least 1.5 mm. Itturns out that particularly good fatigue behavior is achieved if thethickness of the thick metal sheet is between 1.5 and 2.5 mm inclusive.According to the invention, it is also possible to apply more than onethick metal sheet. For instance, more than one metal sheet with athickness of at least 1 mm can be interconnected in the fiber laminateaccording to the invention, creating a composite (thicker) metal sheet.In an embodiment, a laminate according to the invention comprises atleast two thick metal sheets that are interconnected by means of afiber-reinforced polymer layer having a reduced fiber volume content.

The rest of the laminate can in principle be constructed out of anymaterial known to the person skilled in the art. It is thus possible forthe rest of the laminate to comprise a metal sheet or more than onemetal sheet adhesively bonded to each other via adhesive layers that arepossibly reinforced with fibers. In this respect, it is possible to optfor a wide thickness range of the metal sheets in the rest of thelaminate. The rest of the laminate can also comprise a fiber-reinforcedpolymer. In an embodiment, the rest of the laminate comprises metalsheets and fiber-reinforced polymer layers connected thereto, the fibervolume content of which is at least 50 volume-%. With this embodiment,the rest of the laminate substantially corresponds to the laminatealready known in the prior art. Applying such a laminate furtherenhances the mechanical properties, in particular the fatigueresistance.

Yet another embodiment of a laminate according to the invention ischaracterized in that, when the laminate is in unloaded state, acompressive stress on average prevails in the metal sheets of the restof the laminate, and a tensile stress on average in the fiber-reinforcedpolymer layers. It should be noted that the presence of a tensile stressin the fiber-reinforced polymer layers does not mean that these layersonly demonstrate tensile stresses. Rather a tensile stress prevails onaverage in a specific direction. The fiber-reinforced polymer layers canbe subjected to tensile stress by drawing the laminate in a specificdirection, whereby the metal sheets are deformed plastically. Theaverage tensile stress prevailing in this direction in the polymerlayers gives rise to an average compressive stress in the same directionin the metal sheets of the laminate. Fatigue behavior is furtherimproved, by pre-stressing the rest of the laminate and then adhering itto at least one thick metal sheet by means of a fiber-reinforced polymerlayer having a reduced fiber volume content.

It is advantageous to characterize the laminate according to theinvention in that the metal sheets and/or the fiber-reinforced polymerlayers in the rest of the laminate comprise a material that is differentto the thick metal sheets and/or fiber-reinforced polymer layers havinga reduced fiber volume content. In this way it is possible to set theproperties of the metal layers and/or fiber-reinforced polymer layers insuch a way that they are optimal for the function required in thelaminate of the layer in question. It therefore turns out that forexample fatigue resistance is further improved if the laminate accordingto the invention is characterized in that the metal sheets in the restof the laminate have a higher yield stress than the thick metal sheets.It also turns out to be advantageous if the fiber-reinforced polymerlayer in the rest of the laminate positioned closest to a thick metalsheet has a reduced fiber volume content.

In another embodiment, a laminate according to the invention, thethickness of the metal sheets in the rest of the laminate is less than0.8 mm, between 0.2 and 0.8 mm inclusive, between 0.3 and 0.6 mm.Although applying thinner metal sheets per se leads to higher costs andis therefore not naturally obvious, it turns out that applying them inthe rest of the laminate leads to a significant improvement in theproperties of the overall laminate. The laminate according to theinvention is additionally advantageous in that thinner metal sheets onlyhave to be applied in a part of the laminate to be sufficient to achievethese improved properties. The same advantages are achieved if thethickness of the fiber-reinforced polymer layers in the rest of thelaminate is less than 0.8 mm, for example between 0.2 and 0.6 mminclusive.

The fiber-reinforced polymers applied in the fiber metal laminate arelight and strong and comprise reinforcing fibers embedded in a polymer.The polymer also acts as a bonding means between the various layers.Reinforcing fibers that are suitable for use in the fiber-reinforcedpolymer include for example glass fibers, carbon fibers and metalfibers, and if required can also include drawn thermoplastic polymerfibers, such as aramid fibers, PBO fibers (Zylon™), M5™ fibers, andultrahigh molecular weight polyethylene or polypropylene fibers, as wellas natural fibers such as flax, wood and hemp fibers, and/orcombinations of the above fibers. It is also possible to use commingledand/or intermingled rovings. Such rovings comprise a reinforcing fiberand a thermoplastic polymer in fiber form. Examples of suitable matrixmaterials for the reinforcing fibers are thermoplastic polymers such aspolyamides, polyimides, polyethersulphones, polyetheretherketone,polyurethanes, polyethylene, polypropylene, polyphenylene sulphides(PPS), polyamide-imides, acrylonitrile butadiene styrene (ABS),styrene/maleic anhydride (SMA), polycarbonate, polyphenylene oxide blend(PPO), thermoplastic polyesters such as polyethylene terephthalate,polybutylene terephthalate, as well as mixtures and copolymers of one ormore of the above polymers. The thermoplastic polymers further comprisean almost amorphous thermoplastic polymer having a glass transitiontemperature T_(g) of greater than 140° C., for example greater than 160°C., such as polyarylate (PAR), polysulphone (PSO), polyethersulphone(PES), polyetherimide (PEI) or polyphenylene ether (PPE), and inparticular poly-2,6 dimethyl phenylene ether. According to theinvention, it is also possible to apply a semicrystalline orparacrystalline thermoplastic polymer having a crystalline melting pointT_(m) of greater than 170° C., for example greater than 270° C., such aspolyphenylene sulphide (PPS), polyetherketones, in particularpolyetheretherketone (PEEK), polyetherketone (PEK) andpolyetherketoneketone (PEKK), “liquid crystal polymers” such as XYDAR byDartco derived from monomers biphenol, terephthalic acid andhydrobenzoic acid. Suitable matrix materials also comprise thermosettingpolymers such as epoxies, unsaturated polyester resins,melamine/formaldehyde resins, phenol/formaldehyde resins, polyurethanes,etcetera.

In the laminate according to the invention, the fiber-reinforced polymerof one or more layers can comprise substantially continuous fibers thatmainly extend in one direction (so-called UD material). It isadvantageous to use the fiber-reinforced polymer in the form of apre-impregnated semi-finished product. Such a “prepreg” shows generallygood mechanical properties after it has been cured, among other reasonsbecause the fibers have already been wetted in advance by the matrixpolymer. In an embodiment of the laminate according to the invention, atleast a part of the fiber-reinforced polymer layers substantiallycomprises two groups of continuous fibers extending in parallel, eachgroup making the same angle with an intermediary direction. Such a stackof prepregs is also referred to by the person skilled in the art as“angle-ply”. In particular a laminate having fiber-reinforced polymerlayers in the rest of the laminate that substantially comprise twogroups of continuous fibers extending in parallel and forming the sameangle with an intermediary direction, is advantageous—particularly so ifthe laminate is applied in skin panels for an aircraft wing for example.

The fiber metal laminate according to the invention may be obtained byconnecting a number of metal sheets and intermediary fiber-reinforcedpolymer layers to each other by heating them under pressure and thencooling them. If desired, the fiber metal laminate obtained in this waycan be pre-drawn to achieve a favorable state of stress. This laminateis then adhesively bonded to at least one thick metal sheet through themedium of at least one fiber-reinforced polymer layer having a reducedfiber volume content. According to the invention, the layers areconnected in a known way by providing them with a suitable adhesive andthen curing this adhesive at least partially at a suitable temperature.In this respect, the adhesive can be applied separately. It is howeveralso possible for the matrix material of the fiber-reinforced polymer toact as an adhesive between the layers. Fiber metal laminates accordingto the invention possess good specific mechanical properties (propertiesper unit of density), in particular with respect to metals such asaluminum.

In an embodiment of a laminate according to the invention, a laminateaccording to the invention is obtained by adhering at least one firstthick metal sheet to at least one second thick metal sheet through themedium of at least one fiber-reinforced polymer layer having a reducedfiber volume content. According to the prior art, it is not usual toadhere thick metal sheets to each other, for example in cases where oneof the metal sheets to be interconnected is discontinued. With thickmetal sheets displaying a discontinuation, a significant load willinevitably be transferred from the discontinuing metal sheet to theadjacent metal sheet. A discontinuing metal sheet with a relativelylarge thickness transfers a significant concentration of stress to theadjacent metal sheet over a relatively wide area. In a static loadscenario, in which for example there is a requirement for high strength,this may lead to delaminations in the interface between the two metalsheets and/or plastic deformation, in turn leading to a loss of strengthin the continuing metal sheet. If adhesively bonded discontinued metalsheets exceed a specific thickness, crack initiation may also easilyoccur in adjacent metal sheets, impacting on the strength of the overallstructure. The above problem may also occur with metal sheets thatcontinue over the entire dimensions of the structure, such as forexample a wing, albeit to a lesser extent. The laminate according to theinvention resolves the above problem at least partially. This is done inthat by using the fiber-reinforced polymer according to the invention asan interface between two thick metal sheets, the concentration of stressin the adjacent metal sheet is reduced and a slow growth in delaminationwill also occur between the metal sheets, further decreasing the stressconcentration in the adjacent layers. Advantageously, sudden steps inthickness in the adhesively bonded metal sheet construction areprevented, said thickness steps being caused by sudden discontinuationof one of the metal sheets along larger breadths. At the end of a metalsheet, the thickness of the layer in question is advantageously reducedto a relatively small thickness. This can easily be achieved for exampleby milling off the material.

Metals that are particularly appropriate to use include light metals, inparticular aluminum alloys, such as aluminum copper and/or aluminum zincalloys, or titanium alloys. The metal sheets composed of an aluminumalloy can be selected according to the invention from the followinggroup of aluminum alloys, such as types AA (USA) No. 2024, AA (USA) No.7075, AA (USA) No. 7085, AA (USA) No. 7475 and/or AA (USA) No. 6013. Inother respects, the invention is not restricted to laminates using thesemetals, so that if desired other aluminum alloys and/or for examplesteel or another suitable structural metal can be used.

In an embodiment, a laminate according to the invention comprises metalsheets, at least a part of which comprises an aluminum-lithium alloy.Such alloys increase the shearing stiffness of the laminate and are usedin particular in the thick metal sheets. Yet another embodimentcomprises a laminate with metal sheets, at least a part of whichcomprises an aluminum-magnesium-scandium alloy. Such alloys furtherenhance resistance to corrosion, and are used in particular in the thickmetal sheets.

Depending on the intended use and requirements set, the optimum numberof metal sheets can easily be determined by the person skilled in theart. Because it is possible with the invention to use thick metal sheetsin the laminate, the total number of metal sheets for a molding of usualthickness will generally not exceed thirty, although the invention isnot restricted to laminates with a maximum number of metal sheets suchas this. According to the invention, the number of metal sheets isbetween 2 and 20, for example between 2 and 10. Because thicker metalsheets can be used in the laminate according to the invention than knownto date, the number of layers in a molding is significantly less than isknown in the prior art, and it is simpler, quicker and therefore cheaperto produce this molding than a molding based on the known laminate.

In an embodiment of a laminate according to the invention, the laminateis constructed from outside to inside of at least one thick metal sheet,at least one fiber-reinforced polymer layer having a reduced fibervolume content and the rest of the laminate. The rest of the laminatecomprises metal sheets and fiber-reinforced polymer layers connectedthereto, the fiber volume content of which is at least 50 volume-%. Inan embodiment, a laminate is y provided that is constructedsymmetrically from outside to inside. Such a symmetrical embodimentcomprises at least two thick metal sheets on the outside, between whicha central laminate is affixed in the form of a number of metal sheetsand fiber-reinforced polymer layers connected thereto, the fiber volumecontent of which is at least 50 volume-%. The two thick metal sheets areconnected to the central laminate by means of at least onefiber-reinforced polymer layer having a reduced fiber volume content. Bystructuring the laminate according to the invention symmetrically withrespect to a plane through the center of the thickness of the laminate,the laminate is at least partially prevented from warping as a result ofinternal stresses. This embodiment of the inventive laminate is alsoadvantageous in that a large part of the infrastructure currently usedwithin the aircraft industry for adhering aircraft panels can also beused for this material, primarily unchanged. Furthermore, thicknesspatterns can simply be applied in such a material in the manner that iscurrently usual for massive aluminum skins, namely by milling the thickoutermost metal (aluminum) layers. In addition, the fatigue propertiesof the present embodiment meet the requirements set by the aviationindustry for “care-free” materials, while also reducing the number offiber-reinforced layers and the number of metal sheets to be treated andmanipulated. When reference is made in this application to a “care-free”material, it is understood to mean a material in which the fatiguecracks subjected to a fatigue load remain so small that the strength ofthe structure remains greater than the strength required for theapplication. However, in a material that is not sensitive to fatigue,the cracks remain so small that they will not be found using the usualtechniques for inspecting aircraft structures. Although not restrictivefor the invention, a crack will generally grow up to at mostapproximately 100 mm in a “care-free” material in the life of anaircraft (20,000 to 60,000 flights), on the basis of an (artificiallymade) initial crack. This maximum crack length can however vary and isinter alia related to the residual strength of the damaged structure.

The laminates according to the invention are particularly suitable forforming skin sheets for the fuselage and/or wing of an aircraft orspacecraft. The invention also comprises an aircraft or spacecraft, thefuselage and/or wing of which is wholly or partially constructed out ofskin sheets of the laminates according to the invention. In anembodiment of the invention, a skin sheet for example for the wing of anaircraft is formed of a laminate that is structured from outside toinside, and symmetrically, of at least one thick metal sheet, then atleast one fiber-reinforced polymer layer having a reduced fiber volumecontent, and the rest of the laminate comprising in this exemplaryembodiment 5 to 10 metal sheets and 4 to 9 fiber-reinforced polymerlayers connected thereto, the fiber volume content of which is at least50 volume-%. It is however also possible for the rest of the laminate tocomprise fewer layers, for example only two metal sheets with anintermediary fiber-reinforced polymer layer. It is furthermore possiblefor the rest of the laminate to comprise only one thick metal sheet. Ifdesired, such a laminate can comprise thick metal sheets of differentand if desired tapering thickness, for example to enable a thicknesspattern to be milled herein. The wing of an aircraft according to theinvention is provided with such skin sheets, such that the fibers of thefiber-reinforced polymer layers in the rest of the laminatesubstantially comprise two groups of continuous fibers extending inparallel, with each group making an angle with the intermediarydirection that corresponds to the longitudinal direction of the wing. Itis thus advantageous for example to allow at least some of thesereinforcing fibers to extend in a direction forming an angle ofapproximately 45 degrees with the longitudinal direction of the wing.When reference is made in this application to the longitudinal directionof the wing, it is understood to be the direction from fuselage to wingtip. The longitudinal direction makes an angle with the bow directionthat can vary depending on the position of the wings, said bow directioncorresponding to the direction of air flow from the front edge to therear edge of the wing. In fatigue-critical areas of the wing, such asthe root of the wing, this can if desired be reinforced according to theinvention with one or more known laminates (generally referred to as“doublers” in this case), such as Glare™ for example, and/or withlaminates according to the invention. An average reduction in stress isachieved by means of the local thickness structure.

A skin sheet according to the invention which is also particularlysuitable is further locally reinforced by means of at least onelongitudinal stiffener connected thereto via an adhesive layer, alsoreferred to as a “stringer” by the person skilled in the art. Thelongitudinal stiffener can comprise a metal laminate possibly reinforcedwith fibers, such as a laminate according to the invention. In anembodiment, the longitudinal stiffener comprises a laminate according tothe invention that substantially only comprises thick metal sheetsformed out of a flat sheet material or obtained by extruding separatethin-walled profiles that are adhesively bonded to each other viafiber-reinforced polymer layers having a reduced fiber volume content. Asuitable exemplary embodiment comprises for example thick aluminumsheets that are approximately 1.5 mm thick.

In a further embodiment of a skin sheet according to the invention, thelongitudinal stiffener is connected to the skin sheet by means of anadhesive layer comprising a fiber-reinforced polymer. A skin sheet thatis also particularly suitable is constructed out of a laminatecomprising from outside to inside at least one thick metal sheet, atleast one fiber-reinforced polymer layer having a reduced fiber volumecontent and the rest of the laminate, whereby the outermost thick metalsheet comprises a sheet integrally provided with stiffening ribs. Such askin sheet will not generally be constructed symmetrically. The sheetprovided with stiffening ribs can comprise an extruded aluminum sheet,referred to as an “extrusion” by the person skilled in the art. Suchextrusions comprise a flat sheet part substantially provided withstiffening elements, said sheet part being obtained by extruding atubular form and then cutting this open, straightening and milling itand if desired pretreating it for adhesion. According to the invention,a laminate is obtained that in particular demonstrates a high toleranceand that can be produced simply and at a low price.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention will emerge from the followingschematic figures, without otherwise being restricted thereto.

FIG. 1 shows a part of the laminate (referred to in the presentapplication as the rest of the laminate) according to the invention withnine layers,

FIG. 2 shows a laminate according to the invention incorporating therest of the laminate according to FIG. 1,

FIG. 3 shows a cross-section of another embodiment of the laminateaccording to the invention,

FIG. 4 shows the development of crack growth when subjected toalternating load for a 4 mm thick aluminum 2024-T3 sheet and threeembodiments of the laminate according to the invention,

FIG. 5 shows the structure of a skin sheet for an aircraft wing using alaminate according to the invention, and

FIG. 6 finally shows a number of structural embodiments of a skin sheetfor an aircraft wing using a laminate according to the invention.

DETAILED DESCRIPTION

FIG. 2 shows a laminate 1 according to the invention with a total of 11layers. It should be noted that the layer thicknesses shown in thefigures do not necessarily correspond to the actual thickness ratios.Laminate 1 comprises two thick metal layers 2 and 3 made of a suitablealuminum alloy on both outer sides. The core of the laminate 1 is formedby a residual laminate 10, that is connected on either side to thickmetal sheets 2 and 3 by means of one fiber-reinforced polymer layer (4,5) on either side, having a reduced fiber volume content. Although notshown in FIG. 2, if desired it is possible to affix more than one thickmetal sheet (2, 3) over each other, as shown in FIG. 3 for two pairs ofthick metal sheets (2 a, 2 b) and (3 a, 3 b), in order to construct therequired thickness. It is also possible to have the thick metal sheets(2, 3) extend in a crossed fashion to each other, whereby overlappingsheets are beveled at the side edges and are positioned so that they areat least partially overlapping along the beveling (a technique referredto as “splicing”). More than one thick metal sheet (2 a, 2 b) and (3 a,3 b) are preferably interconnected by means of fiber-reinforced polymerlayers (4 a, 5 a) having a reduced fiber volume content, as shown inFIG. 3. The connection between the thick metal sheets (2 b, 3 b) and therest 10 of the laminate is formed by fiber-reinforced polymer layers (4b, 5 b) having a reduced fiber volume content. It is possible for thelayers (4 a, 5 a) and (4 b, 5 b) to be formed differently.

FIG. 1 shows an embodiment of the rest 10 of the laminate according tothe invention in the form of a flat rectangular sheet. In the exemplaryembodiment shown, the residual laminate 10 comprises the part of thelaminate 1 not composed of the thick metal sheets (2, 3) and connectinglayers (4, 5). Residual laminate 10 is constructed out of five metalsheets 12 having a thickness of for example 0.2 mm, comprising analuminum alloy, for example 2024-T3. The metal sheets 12 are securelyinterconnected by means of four fiber-reinforced polymer layers 11 basedon epoxy resin that is also a good metal adhesive. A fiber-reinforcedconnecting layer 11 comprises and is formed of glass fibers impregnatedwith the specified polymer, having a fiber volume content ofapproximately 60 vol.-%. These preimpregnated prepregs 11 with athickness of approximately 0.25 mm are formed of (unidirectional) glassfibers extending parallel to each other in direction 13. The residuallaminate 10 is produced by applying the specified layers 11 and 12 toeach other in the sequence shown in FIG. 1, for example on a flat mold.After lamination, the overall structure is cured at a temperaturesuitable for the epoxy resin. For most applications, an epoxy resin witha high glass transition temperature will be most suitable. Such epoxyresins are generally cured at a temperature of approximately 175° C.Although not essential for the invention, it is advantageous, after thestructure shown in FIG. 1 has been cured, to impose an elongation in thestructure's longitudinal direction that is greater than the elasticelongation of the metal sheets 12 and less than the elongation at breakof the fiber-reinforced polymer layers 11. Such a pre-stress of theresidual laminate 10 can for example be achieved by imposing anelongation .epsilon. of between 0.1 and 2 percent on the residuallaminate 10 in the longitudinal direction thereof. Depending on thefibers applied in the fiber-reinforced polymer layers, the range of thiselongation can also lie elsewhere. The residual laminate 10 can bepre-stressed by feeding it through a rolling mill under pressure. Inthis way, a method is provided that can be applied on an industrialscale. By setting the exerted compressive force at a sufficiently highlevel, the deformations in the plane of the residual laminate 10 are ofsuch a size that the imposed elongation .epsilon. in the longitudinaldirection exceeds the plasticity limit of the metal of the metal sheets12, causing the metal sheets 12 to permanently deform, without leadingto a failure of the fiber-reinforced polymer layers 11. By drawing theresidual laminate 10 in the longitudinal direction, a particularlyfavorable state of stress is created, with a compressive stress beingpresent on average in each metal sheet 12 in an unloaded state, and atensile stress being present on average in each fiber-reinforced polymerlayer 11. It will thus be possible for the sublaminate and/or laminatethereby obtained to demonstrate for example an increased yield stress.This is also advantageous for the fatigue behavior. The increased yieldstress is additionally advantageous if types of aluminum are alsoapplied in the laminate that already intrinsically demonstrate anincreased yield stress compared to the known aluminum alloys based oncopper and zinc, such as the 2000 series for example.

FIGS. 5( a), 5(b) and 5(c) show three embodiments of a skin sheet 30 forthe wing of an aircraft. Skin sheet 30 is formed out of a laminate 1according to the invention, comprising a centrally affixed residuallaminate 10, constructed out of metal sheets that are securelyinterconnected by means of fiber-reinforced polymer layers based onepoxy resin, having a fiber volume content of at least 50 volume-%. Thethree skin sheets 30 shown comprise, on the upper side (according to thefigure), a thick metal sheet in the form of a sheet 32 integrallyprovided with stiffening ribs 35, preferably an extrudedaluminum-containing sheet. In the embodiment shown in FIG. 5( a), theunder side is formed of a thick metal sheet 33. Both thick sheets 32 and33 are connected to the centrally positioned residual laminate 10 bymeans of two fiber-reinforced polymer layers 36 having a reduced fibervolume content. In the embodiment shown in FIG. 5( b), the under side ofthe skin sheet 30 (which obviously corresponds to the side of the wingfacing outward) is formed by a number of thick metal layers 34 that areconnected to each other as well as to the central residual laminate 10by means of fiber-reinforced polymer layers 36 having a reduced fibervolume content. In the embodiment shown in FIG. 5( c), the under side ofthe skin sheet 30 is formed by a number of thick metal layers 34 thatare interrupted and connected in a crossed fashion with respect to eachother (partially overlapping) by means of fiber-reinforced polymerlayers 36 having a reduced fiber volume content. However it is alsoadvantageous to position the edges of the thick metal sheetssubstantially against each other (“butted edges”), so that lessstringent requirements have to be set with respect to positioningaccuracy.

FIGS. 6( a) to 6(i) inclusive show nine embodiments of a skin sheet 30for the wing of an aircraft. The reference figures are only indicatedonce in these figures. In these embodiments, skin sheet 30 is reinforcedwith a number of longitudinal stiffeners 40 connected thereto by meansof an adhesive layer 41. Longitudinal stiffeners 40 can comprisealuminum profiles, but preferably comprise a laminate according to theinvention, whereby the adhesive layer 41 preferably comprises afiber-reinforced polymer. In such an exemplary embodiment of thelongitudinal stiffener 40, said stiffener therefore comprises at leastone thick metal sheet, preferably an aluminum sheet, and at least onefiber-reinforced polymer layer having a reduced fiber volume content,which is used to adhesively bond the thick metal sheet to the rest ofthe longitudinal stiffener 40. The rest of said stiffener can then beconstructed of more than one thick metal sheet adhesively bonded to eachother by means of a fiber-reinforced polymer layer having a low fibervolume content if desired; or a known laminate, for example Glare™; or acombination of thick metal sheets and a known laminate, for exampleGlare™. It is also possible to construct the longitudinal stiffenersubstantially out of thick metal sheets formed out of a flat sheetmaterial or obtained by extruding separate thin-walled profiles that areadhesively bonded to each other via fiber-reinforced polymer layershaving a reduced fiber volume content. The skin sheet 30 is furtherformed out of a laminate 1 according to the invention, comprising acentrally affixed residual laminate 10, constructed out of metal sheetsthat are securely interconnected by means of fiber-reinforced polymerlayers based on epoxy resin, having a fiber volume content of at least50 volume-%. The skin sheet 30 shown in FIG. 6( a) comprises a thickmetal sheet (42, 43) on either side of the centrally affixed residuallaminate 10. Both thick sheets (42, 43) are connected to the centrallypositioned residual laminate 10 by means of two fiber-reinforced polymerlayers 46 having a reduced fiber volume content. In the embodimentsshown in FIGS. 6( b), 6(c) and 6(d), the under and/or upper side of theskin sheet 30 is formed by a number of thick metal layers 44 that areconnected to each other as well as to the central residual laminate 10by means of fiber-reinforced polymer layers 46 having a reduced fibervolume content. In the embodiments shown in FIGS. 6( e) to 6(i)inclusive, the under and/or upper side of the skin sheet 30 is formed bya number of thick metal layers 44 that are interrupted and connected ina crossed fashion with respect to each other (possibly partiallyoverlapping) by means of fiber-reinforced polymer layers 46 having areduced fiber volume content. Skin sheets of wings are generally of athickness varying between 3 mm at the wing tip and a maximum ofapproximately 30 mm at the wing root. With the known laminate, such athickness can only be achieved by making a structure of approximately 50aluminum sheets and 49 layers of fiber-reinforced polymer, assuming athickness of 3 mm in a 5/4 construction (5 layers of aluminum sheet and4 layers of fiber-reinforced polymer in-between). It should be notedthat stacking such numbers, in particular the metal sheets, constitutesan almost unworkable production process, with a view to manipulating themany thin sheets and pretreating them. The laminate according to theinvention resolves this problem.

A number of laminates 1 according to the invention were subjected to afatigue test under a load extending in the direction 13. In FIG. 4, thefatigue behavior of the laminates 1 according to the invention is shownand compared with the fatigue behavior of a 4 mm thick aluminum sheet oftype 2024-T3. To this end, testpieces of 200×500 mm of the laminateswere subjected to tensile stress with a sinusoidally extending averageload of 100 MPa and a frequency of 10 Hz. The testpieces were beforehandprovided with a sharp initial crack transverse to the direction oftension having a length of 2 a=10 mm. In FIG. 4, the half crack length aexpanded along the vertical axis. The total number of flights 50 of thesimulated wing load expanded along the horizontal axis. [0036] Line Icorresponds to the crack growth of the 4 mm thick aluminum sheet of type2024-T3. [0037] Line II corresponds to the crack growth of a laminatecomprising a central residual laminate made of Glare™ fiber metallaminate adhesively bonded in a second autoclave cycle between two 4 mmthick aluminum 2024-T3 sheets, using FM 94 adhesive film (withoutfibers). The adhesive film was provided with a carrier that maintainedthe thickness of the adhesive layer during the autoclave cycle. Thecentral residual laminate comprises a Glare™ 1-5/4-0.4 laminate (basedon five 0.4 mm thick aluminum layers) that was pre-drawn with anelongation of 0.5%. A standard S2 glass prepreg based on FM 94 adhesivewas used in this Glare™ fiber metal laminate. The fiber volume contentof the prepreg was 60 volume-%. [0038] Line III corresponds to the crackgrowth of a laminate 1 constructed in the same way as the line IIlaminate, provided that the central residual laminate was adhesivelybonded on the outermost 4 mm thick aluminum 2024-T3 layers using thesame S2 glass prepreg as applied in the central laminate.

Finally, line IV corresponds to a laminate according to the invention.This laminate is constructed in the same way as the laminatecorresponding to line III, provided that a prepreg based on S2 glassfiber and FM 94 epoxy, combined with an FM 94 adhesive film, was appliedin the second autoclave cycle to adhere between the 4 mm thick aluminumsheets and the central fiber metal laminate. Thus on balance, thisadhesive layer showed a reduced fiber volume.

As can be seen in FIG. 4, the laminate 1 (line IV) according to theinvention demonstrates clearly reduced crack growth when subjected tothe specified load in comparison with the aluminum laminate (line I) orother laminates (lines II and III) according to the prior art.

Wherever reference is made in the description and claims to the modulusof elasticity, tensile strength and elongation at break of the fibers,they are understood to mean the values under tensile load in thelongitudinal direction of the fiber and are determined via measurementson the completed laminate.

Within the scope of the invention, various changes can be incorporated.Although metal sheets of the same thickness are firstly applied in thelaminates according to the invention, it is in principle also possibleto apply metal sheets having two or more different thicknesses in oneand the same laminate in a possibly symmetrical stack. In general, thethickness of the polymer layer between two consecutive metal sheets inthe residual laminate will approximately be of the same size order asthat of each of the metal sheets. If desired, the laminates canfurthermore demonstrate a tapering thickness as well as a taperingdepth.

1. A structure comprising: at least a first metal layer having aconstant thickness of at least about 1.5 mm; and at least a second metallayer having a constant thickness of at least about 1.5 mm, wherein thefirst metal layer and the second metal layer are connected to each otherby a fiber-reinforced epoxy prepreg having a fiber volume content notexceeding about 39%.
 2. The structure of claim 1 wherein thefiber-reinforced epoxy prepreg includes reinforcing fibers embedded in apolymer matrix.
 3. The structure of claim 2 wherein the reinforcingfibers are selected from the group consisting of glass fibers, carbonfibers, metal fibers, drawn thermoplastic fibers, natural fibers andcombinations thereof.
 4. The structure of claim 2 wherein the polymermatrix is a thermoplastic polymer matrix or a thermosetting polymermatrix.
 5. The structure of claim 1 wherein the thickness of the firstmetal layer and the second metal layer is about 2.5 mm.
 6. The structureof claim 2 wherein the reinforcing fibers are impregnated with thepolymer matrix in a partially cured state.
 7. The structure of claim 2wherein the reinforcing fibers are substantially continuous and extendin one direction.
 8. The structure of claim 2 wherein the reinforcingfibers are present in two groups that are substantially continuous andextend in parallel, each group making an identical angle with anintermediary direction.
 9. The structure of claim 1 wherein the firstmetal layer and the second metal layer include an aluminum alloy. 10.The structure of claim 1 wherein the first metal layer and the secondmetal layer include an aluminum-lithium alloy.
 11. The structure ofclaim 1 wherein the first metal layer and the second metal layer includean aluminum-magnesium-scandium alloy.